1. Field of the Invention
The present invention relates to a positive electrode active material that can improve battery performance such as high-rate capability, a method of manufacturing the positive electrode active material, and a non-aqueous electrolyte secondary battery using the positive electrode active material.
2. Description of Related Art
Mobile information terminal devices such as mobile telephones, notebook computers, and PDAs have become smaller and lighter at a rapid pace in recent years. This has led to a demand for higher capacity batteries as the drive power source for the mobile information terminal devices. With their high energy density and high capacity, non-aqueous electrolyte secondary batteries, which perform charge and discharge by transferring lithium ions between the positive and negative electrodes, have been widely used as a driving power source for the mobile information terminal devices.
As the mobile information terminal devices tend to have greater numbers of functions, such as moving picture playing functions and gaming functions, the power consumption of the devices tends to increase. It is therefore strongly desired that the non-aqueous electrolyte secondary batteries used for the power sources of such devices have further higher capacities and higher performance to achieve longer battery life and improved output power. In addition, applications of the non-aqueous electrolyte secondary batteries are expected to expand from not just the above-described applications but to power tools, power assisted bicycles, and moreover HEVs. In order to meet such expectations, it is strongly desired that the capacity and the performance of the battery be improved further.
In order to increase the capacity of the non-aqueous electrolyte secondary battery, it is essential to increase the capacity of the positive electrode. A lithium-excess lithium-transition metal composite oxide represented by the chemical formula Li1+xMn1−x−yMyO2, where M is at least one transition metal other than manganese, has been proposed as a positive electrode material. This material is known to show a high discharge capacity, a maximum of 270 mAh/g. (See Electrochemical and Solid-State Letters 9(5), A221-A224, (2006).) The just-mentioned oxide, however, has limited practical applications because it is poor in high-rate capability, cycle performance, initial charge-discharge efficiency, and the like. It is generally known that the high-rate capability is affected by the lithium diffusion rate in the bulk and the smoothness of lithium insertion and deinsertion in the particle surface. It appears, therefore, conceivable that the high-rate capability may be improved by reducing the particle size of the positive electrode active material so that the diffusion distance in the particle can decrease. However, when the particle size is reduced, another problem arises that the packing density of the positive electrode active material decreases and consequently the energy density decreases.
In view of such circumstances, the following proposals have been made.
Proposal (1): Using a lithium-excess lithium-transition metal composite oxide the surface of which is coated with Al2O3 or LiNiPO4 as the positive electrode active material (see Electrochemical and Solid-State Letters, 9(5), A221-A224 (2006) and Electrochemistry Communications, 11(4), pp. 748-751 (2009)).
Proposal (2): Coating the surface of a positive electrode active material having a layered structure with a positive electrode active material having a spinel structure such as LiMn2O4 (see Japanese Published Unexamined Patent Application No. 2009-129721).
Proposal (3): Acid-treating a lithium-excess lithium-transition metal composite oxide using an acid such as HNO3 to remove excess lithium from the positive electrode active material, in order to modify the surface of the active material (see Journal of The Electrochemical Society 153(6), A1186-A1192, (2006)).
Proposal (4): Dry-blending a nickel-based positive electrode active material and (NH4)2SO4 and thereafter heat-treating the material at 700° C. (see Japanese Published Unexamined Patent Application No. 2009-146739).
Problem with Proposal (1)
According to proposal (1), the cycle performance may be improved somewhat. However, since the positive electrode active material having a spinel structure does not exist on the surface, the high-rate capability becomes poor, or even if improved, significant improvement is impossible.
Problem with Proposal (2)
According to proposal (2), the positive electrode active material having a layered structure in the inside and the positive electrode active material having a spinel structure on the outside have different structures from each other, and moreover, the compositions of the positive electrode active materials are also different from each other. As a consequence, a boundary forms between the positive electrode active material in the inside and the positive electrode active material on the surface, and this boundary limits the diffusion of lithium in the positive electrode active material particle. For this reason, it is impossible to improve the high-rate capability dramatically.
Problem with Proposal (3)
According to proposal (3), the initial charge-discharge efficiency can be improved by the effect of modifying the positive electrode active material surface. However, since the positive electrode active material surface is damaged by the acid, the cycle performance degrades.
Problem with Proposal (4)
When proposal (4) is applied to a manganese-based positive electrode active material such as that in the present invention, a decrease in discharge capacity and degradations in charge-discharge efficiency and high-rate capability occur because of the different in the structure of the positive electrode active material and the high heating temperature.